Aging of the hematopoietic system is associated with decline in B lymphopoiesis and altered B cell repertoire, which compromise the adaptive immunity. We identified MEF2C as a novel transcriptional regulator that protects bone marrow (BM) B lymphoid progenitors from prematurely occurring aging-like phenotype. We will investigate the hypothesis that MEF2C protects B lymphopoiesis by regulating of DNA repair and cell cycle checkpoint machinery, which are critical for B cell development and BCR rearrangement. By ablating MEF2C in the hematopoietic system, we identified a requirement for MEF2C in promoting survival of B cell progenitors, most notably pre-B cells, which is a major bottleneck during B cell development and becomes severely compromised during aging. Irradiation assay revealed heightened requirement for MEF2C to promote B cell progenitor survival during regenerative stress. Genome-wide gene expression analysis in pre-B cells suggested that MEF2C enhances the expression of factors involved in DNA repair and BCR rearrangement machinery, which was supported by preliminary data from DNA damage and recombination assays. Our goal now is to elucidate the cellular and molecular mechanisms by which loss of a single gene, MEF2C, results in very similar phenotype as observed during aging, and assess whether aged B cell progenitors become resistant to MEF2C, or MEF2C downstream effectors. In Aim 1, we will examine the function of MEF2C in regulating DNA damage response, cell cycle and BCR rearrangement through key transitions of B cell progenitor development, and investigate if loss of MEF2C and advanced age similarly correlate with inefficiency in these processes. Furthermore, these studies enable direct comparison of how genotoxic/regenerative stress that occurs during recovery of B lymphoid compartment from irradiation modifies the effects of MEF2C deficiency and defects observed during physiological aging. In Aim 2 we will define the direct transcriptional targets of MEF2C in pre-B cells using ChIP-sequencing and RNA-sequencing. Moreover, as MEF2C fails to prevent aging associated defects in old B cell progenitors despite persistent high level of MEF2C expression, we will investigate if defective B lymphopoiesis in the aged is influenced by altered binding of MEF2C to its transcriptional targets, or due to a resistance of the aged B lymphoid cells to MEF2C regulated pathways. These studies include analysis of active histone marks at MEF2C regulated enhancers and promoters upon MEF2C loss and during aging to investigate if MEF2C and/or age-dependent factors modify the epigenetic state of these B cell regulatory genes. By studying how MEF2C regulated transcriptional networks change during aging, we seek to uncover novel mechanisms contributing to the functional defects in the aged B cell progenitors, and provide new insights for developing strategies to protect and rejuvenate the aging B lymphoid system.